1. Field of the Invention
The present invention relates to a single crystal production method based on the Czochralski method and more specifically, to a method for preventing a crystal from being dislocated, becoming polycrystalline and being deformed due to waves on the surface of a melt.
2. The Prior Art
While crystals of various element semiconductors and compound semiconductors have been used as a substrate for making electronic devices and photoelectric devices, the Czochralski method is used for producing these crystals. The Czochralski method is useful for producing large crystals, and most silicon single crystals in the current semiconductor device fabrication are produced by means of the Czochralski method.
FIG. 1 is a longitudinal sectional view of a crystal puller as a model, based on the Czochralski method. In this crystal puller, a melt 4 is obtained by filling with the crystal raw material a crucible 2 housed in a susceptor 1 disposed on a crucible axis 7 that can be revolved as well as rise and fall, and by heating the crystal raw material with a heater 3 to thereby melt the crystal raw material. The puller comprises, on its top, a wire 21 and a driving mechanism 20 for revolving and pulling up the crystal. A seed crystal 5 hung on the wire is dipped in the melt 4 and thereafter pulled up with the seed crystal 5 and the crucible 2 revolved, to thereby obtain a crystal 6. Since temperature becomes high inside the puller, the puller body 9 is protected by thermal insulation materials 8 and 10.
However, the stability of the surface of the melt greatly influences the stability of growth of a single crystal, because, according to the Czochralski method, a single crystal is grown from the free surface of the melt. Namely, the surface of the melt may be waved due to mechanical vibrations, the crystal may be dislocated or become polycrystalline due to increase of the temperature fluctuations accompanying the waves, so that the crystal cannot be used as a substrate for a semiconductor device. And, even if the crystal can avoid these problems, the crystal is likely to undergo deformation of the crystal shape and the product yield may be lowered because the crystal does not take a predetermined shape. Further, in case of a melt with a low viscosity, e.g. silicon and in case a large quantity of a melt is used in a large crucible, once large waves occur, the waves are not easily settled because the inertia of the melt is large with respect to the viscosity.
Accordingly, as to the single crystal production apparatus based on the Czochralski method, it is suggested that the rotation axes of the crucible and the crystal should be adjusted in a vertical direction in order to prevent the melt from being waved due to the rotation of the crystal and the crucible; the revolution central axes of the crucible and the crystal should be adjusted to accord with each other as much as possible, and the vibrations from the driving system relating to the rotation and the vertical movement of the crucible and the crystal are not transmitted to the melt (see Japanese Patent Application Disclosure Nos. 2000-169290 and 2000-86387).
As described above, thorough countermeasures against the driving system are employed to be able to settle waves on the surface of the melt that permanently occur during the single crystal production. However, in many cases, many waves occurring during the actual crystal pulling-up have a characteristic of suddenly starting to occur during the single crystal growing and being settled after a certain time. For this kind of waves on the surface of the melt, it is clear that the improvement of precision in setting the driving system and the countermeasures against the vibration transmission system are not clearly successful as a direct solution, and they are not very effective in fact.